The invention concerns circuitry for correcting geometrical distortions while reproducing signals on the screen of a cathode-ray tube in a display monitor with a device for coupling a line-frequency correction current into the switching circuit that deflects the electron beams vertically.
Since the color tubes in display monitor are now being provided while they are being manufactured with means of deflecting the electron beams, the deflection unit must be very precisely adjusted in relation to the picture tube during assembly. The optimal position of the deflection unit at the neck of the tube must be adapted to color purity, pixel focus, and convergence. Since the beam system in a color tube has unavoidable tolerances, the optimal position of the deflection unit can only be a compromise between the requisite adjustments. Many tube units accordingly have one of these inherent errors, characterized in that the horizontal lines are at a slight angle to the horizontal reference grid, meaning that the horizontal are skewed around points on the vertical midline of the picture, so that the exhibit a slight tendency to rise and fall. An angular error of this type is especially apparent when characters are being displayed.
Circuits that eliminate geometrical distortions in the form of pillow distortions on the screen are known. A line-frequency correction current is coupled into the vertical-deflection circuit in this process. To correct this specific type of distortion, the line-frequency current must first be converted into a parabolic shape, which is expensive (German App. 1 916 104).